The present invention relates to a battery charging apparatus and a battery charging method for charging rechargeable batteries including alkali electrolytic solution type secondary batteries, such as nickel-cadmium batteries and nickel-hydrogen batteries.
A conventionally known method for detecting a full-charge condition of a nickel-cadmium battery or the like is performed by sampling a battery voltage at predetermined intervals during a charging operation. The value of the sampled battery voltage increases in accordance with the progress of the charging operation. This maximum value (VMAX) is memorized after finishing every sampling. Then, a newly sample battery voltage (V) is compared with the memorized maximum value. When the difference (.DELTA.V=VMAX-V) is equal to or larger than a predetermined value (.delta.), it is regarded that the battery voltage value showed a decline after reaching a peak. That is, when the battery is fully charged, the battery voltage (V) causes a predetermined voltage drop from the peak value (VMAX). Upon detecting this characteristic decline of the battery voltage (.DELTA.V.gtoreq..delta.), the charging operation is stopped. This conventional charging method is referred to as a "-.DELTA.V charge control method" and, for example, disclosed in the U.S. Pat. No. 4,998,057.
Generally, this kind of monitoring operation for detecting the battery charging condition is realized by a microcomputer. The battery voltage is A/D converted when it is processed by the microcomputer.
However, when a sampled battery voltage value is near a threshold of an A/D converter, a converted digital value possibly fluctuates or hunts between two digital values due to a quantization error. For example, when a resolution is 256 bits/5 V full scale, an input voltage may have a value in the vicinity of 128.5/256*5 V. In this case, the converted digital value fluctuate between 128 and 129.
If the above-described predetermined value (.delta.) is 1 bit, this will result in a failure in the judgement of the charging condition.
To avoid this malfunction, the predetermined value (.delta.) must have a larger margin equal to or larger than 2. However, setting a large margin in the charging operation is not preferably in that the sensitivity in the detection of the fully-charged condition is worsened. Needless to say, the lowered sensitivity results in an excessive charging. The cycle life of the battery will be shortened undesirably.